gh-104504: cases generator: Add --warn-unreachable to the mypy config

Tools/cases_generator/_typing_backports.py

@@ -0,0 +1,15 @@
+"""Backports from newer versions of the typing module.
+
+We backport these features here so that Python can still build
+while using an older Python version for PYTHON_FOR_REGEN.
+"""
+
+from typing import NoReturn
+
+
+def assert_never(obj: NoReturn) -> NoReturn:
+    """Statically assert that a line of code is unreachable.
+
+    Backport of typing.assert_never (introduced in Python 3.11).
+    """
+    raise AssertionError(f"Expected code to be unreachable, but got: {obj}")

Tools/cases_generator/analysis.py

@@ -2,6 +2,7 @@
 import sys
 import typing
 
+from _typing_backports import assert_never
 from flags import InstructionFlags, variable_used
 from formatting import prettify_filename, UNUSED
 from instructions import (
@@ -172,7 +173,7 @@ def parse_file(self, filename: str, instrs_idx: dict[str, int]) -> None:
                     self.pseudos[name] = thing
                     self.everything.append(thing)
                 case _:
-                    typing.assert_never(thing)
+                    assert_never(thing)
         if not psr.eof():
             raise psr.make_syntax_error(f"Extra stuff at the end of {filename}")
 
@@ -368,7 +369,7 @@ def analyze_macro(self, macro: parsing.Macro) -> MacroInstruction:
                         # SAVE_IP in a macro is a no-op in Tier 1
                         flags.add(instr.instr_flags)
                 case _:
-                    typing.assert_never(component)
+                    assert_never(component)
         format = "IB" if flags.HAS_ARG_FLAG else "IX"
         if offset:
             format += "C" + "0" * (offset - 1)
@@ -409,5 +410,5 @@ def check_macro_components(
                 case parsing.CacheEffect():
                     components.append(uop)
                 case _:
-                    typing.assert_never(uop)
+                    assert_never(uop)
         return components

Tools/cases_generator/generate_cases.py

@@ -13,6 +13,7 @@
 from collections.abc import Iterator
 
 import stacking  # Early import to avoid circular import
+from _typing_backports import assert_never
 from analysis import Analyzer
 from formatting import Formatter, list_effect_size
 from flags import InstructionFlags, variable_used
@@ -154,8 +155,8 @@ def effect_str(effects: list[StackEffect]) -> str:
             return str(n_effect)
 
         instr: AnyInstruction | None
-        popped: str | None
-        pushed: str | None
+        popped: str | None = None
+        pushed: str | None = None
         match thing:
             case parsing.InstDef():
                 if thing.kind != "op" or self.instrs[thing.name].is_viable_uop():
@@ -171,7 +172,6 @@ def effect_str(effects: list[StackEffect]) -> str:
                 popped, pushed = stacking.get_stack_effect_info_for_macro(instr)
             case parsing.Pseudo():
                 instr = self.pseudo_instrs[thing.name]
-                popped = pushed = None
                 # Calculate stack effect, and check that it's the the same
                 # for all targets.
                 for target in self.pseudos[thing.name].targets:
@@ -189,7 +189,7 @@ def effect_str(effects: list[StackEffect]) -> str:
                         assert popped == target_popped
                         assert pushed == target_pushed
             case _:
-                typing.assert_never(thing)
+                assert_never(thing)
         return instr, popped, pushed
 
     @contextlib.contextmanager
@@ -379,7 +379,7 @@ def write_metadata(self, metadata_filename: str, pymetadata_filename: str) -> No
         # Compute the set of all instruction formats.
         all_formats: set[str] = set()
         for thing in self.everything:
-            format: str | None
+            format: str | None = None
             match thing:
                 case OverriddenInstructionPlaceHolder():
                     continue
@@ -388,7 +388,6 @@ def write_metadata(self, metadata_filename: str, pymetadata_filename: str) -> No
                 case parsing.Macro():
                     format = self.macro_instrs[thing.name].instr_fmt
                 case parsing.Pseudo():
-                    format = None
                     for target in self.pseudos[thing.name].targets:
                         target_instr = self.instrs.get(target)
                         assert target_instr
@@ -398,7 +397,7 @@ def write_metadata(self, metadata_filename: str, pymetadata_filename: str) -> No
                             assert format == target_instr.instr_fmt
                     assert format is not None
                 case _:
-                    typing.assert_never(thing)
+                    assert_never(thing)
             all_formats.add(format)
 
         # Turn it into a sorted list of enum values.
@@ -488,7 +487,7 @@ def write_metadata(self, metadata_filename: str, pymetadata_filename: str) -> No
                                 self.pseudo_instrs[thing.name]
                             )
                         case _:
-                            typing.assert_never(thing)
+                            assert_never(thing)
 
             with self.metadata_item(
                 "const struct opcode_macro_expansion "
@@ -525,7 +524,7 @@ def write_metadata(self, metadata_filename: str, pymetadata_filename: str) -> No
                         case parsing.Pseudo():
                             pass
                         case _:
-                            typing.assert_never(thing)
+                            assert_never(thing)
 
             with self.metadata_item(
                 "const char * const _PyOpcode_uop_name[OPCODE_UOP_NAME_SIZE]", "=", ";"
@@ -774,7 +773,7 @@ def write_instructions(
                     case parsing.Pseudo():
                         pass
                     case _:
-                        typing.assert_never(thing)
+                        assert_never(thing)
 
         print(
             f"Wrote {n_instrs} instructions and {n_macros} macros "
@@ -818,7 +817,7 @@ def write_executor_instructions(
                     case parsing.Pseudo():
                         pass
                     case _:
-                        typing.assert_never(thing)
+                        assert_never(thing)
         print(
             f"Wrote {n_instrs} instructions and {n_uops} ops to {executor_filename}",
             file=sys.stderr,
@@ -850,7 +849,7 @@ def write_abstract_interpreter_instructions(
                     case parsing.Pseudo():
                         pass
                     case _:
-                        typing.assert_never(thing)
+                        assert_never(thing)
         print(
             f"Wrote some stuff to {abstract_interpreter_filename}",
             file=sys.stderr,

Tools/cases_generator/mypy.ini

@@ -2,13 +2,12 @@
 files = Tools/cases_generator/
 pretty = True
 
+# Make sure Python can still be built
+# using Python 3.10 for `PYTHON_FOR_REGEN`...
 python_version = 3.10
 
-# Be strict:
+# ...And be strict:
 strict = True
 strict_concatenate = True
 enable_error_code = ignore-without-code,redundant-expr,truthy-bool
-
-# Don't enable this one yet;
-# it has a lot of false positives on `cases_generator`
-warn_unreachable = False
+warn_unreachable = True